Iodide can function as an antioxidant as it is a reducing species that can detoxify reactive oxygen species such as hydrogen peroxide. Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are the ancestors of multicellular eukaryotic algae (1). Algae that contain the highest amount of iodine (1-3 % of dry weight) and peroxidase enzymes, were the first living cells to produce poisonous oxygen in the atmosphere. Therefore algal cells required a protective antioxidant action of their molecular components, in which iodides, through peroxidase enzymes, seem to have had this specific role. In fact, iodides are greatly present and available in the sea, where algal phytoplankton, the basis of marine food-chain, acts as a biological accumulator of iodides, selenium, (and n-3 fatty acids) :; Antioxidant biochemical mechanism of iodides, probably one of the most ancient mechanisms of defense from poisonous reactive oxygen species:; An iodide ion is an iodine atom with a -1 charge. Compounds with iodine in formal oxidation state -1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide. This is the same naming scheme as is seen with chlorides and bromides. The chemical test for an iodide compound is to acidify the aqueous compound by adding some drops of acid, to dispel any carbonate ions present, then adding lead(II) nitrate, yielding a bright yellow precipitate of lead iodide. Most ionic iodides are soluble, with the exception of yellow silver iodide and yellow lead iodide. Aqueous solutions of iodide dissolve iodine better than pure water due to the formation of complex ions:.

Responsible for the deiodination of T4 (3,5,3',5'-tetraiodothyronine) into T3 (3,5,3'-triiodothyronine) and of T3 into T2 (3,3'-diiodothyronine). Plays a role in providing a source of plasma T3 by deiodination of T4 in peripheral tissues such as liver and kidney.

Responsible for the deiodination of T4 (3,5,3',5'-tetraiodothyronine) into T3 (3,5,3'-triiodothyronine). Essential for providing the brain with appropriate levels of T3 during the critical period of development.

Catalyzes the oxidative NADPH-dependent deiodination of monoiodotyrosine (L-MIT) or diiodotyrosine (L-DIT). Acts during the hydrolysis of thyroglobulin to liberate iodide, which can then reenter the hormone-producing pathways. Acts more efficiently on monoiodotyrosine than on diiodotyrosine.

Responsible for the deiodination of T4 (3,5,3',5'-tetraiodothyronine) into RT3 (3,3',5'-triiodothyronine) and of T3 (3,5,3'-triiodothyronine) into T2 (3,3'-diiodothyronine). RT3 and T2 are inactive metabolites. May play a role in preventing premature exposure of developing fetal tissues to adult levels of thyroid hormones. Can regulate circulating fetal thyroid hormone concentrations throughout gestation. Essential role for regulation of thyroid hormone inactivation during embryological development.